L10_digital_modulation_02

L10_digital_modulation_02 - Digital Modulation(Part II...

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1 6.02 Spring 2008 Lecture 10 Digital Modulation (Part II), Slide 1 Digital Modulation (Part II) Receiver noise vs. intersymbol interference (ISI) Raised Cosine Filter Complex mixing for frequency offset removal Copyright © 2007 by M.H. Perrott & C. G. Sodini All rights reserved. 6.02 Spring 2008 Lecture 10 Digital Modulation (Part II), Slide 2 Review of Digital I/Q Modulation Leverage analog communication channel to send discrete-valued symbols Example: send symbol from set {-3,-1,1,3} on both I and Q channels each symbol period At receiver, sample I/Q waveforms every symbol period Associate each sampled I/Q value with symbols from a set {-3,-1,1,3} on both I and Q channels
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2 6.02 Spring 2008 Lecture 10 Digital Modulation (Part II), Slide 3 Transmit and Receiver Filters Transmit filter examined last time Tradeoff of transmitted bandwidth vs intersymbol interference (ISI) Receive filter examined this time Previously assumed to have very wide bandwidth so as not to influence ISI 6.02 Spring 2008 Lecture 10 Digital Modulation (Part II), Slide 4 Tools for ISI Examination Eye diagram Shows transition behavior between symbols ISI causes closing of eye Constellation diagram Shows aggregate placement of sampled I/Q values ISI causes spreading of symbol points
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3 6.02 Spring 2008 Lecture 10 Digital Modulation (Part II), Slide 5 Impact of Receiver Noise Receiver noise adds to desired I/Q signals and causes corruption Eye diagram reveals closing of eye Constellation reveals high spread in symbol points Key insight: lowering the receive filter bandwidth improves rejection of noise 6.02 Spring 2008 Lecture 10 Digital Modulation (Part II), Slide 6 Impact of Lower Receive Filter Bandwidth Receiver noise is assumed to be
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